


Beautiful Together

by Dragonmaster



Series: Beautiful Together [1]
Category: Transformers: Prime, Undertale (Video Game)
Genre: Crack Crossover, Crossover, Crossover Pairings, M/M, One Shot, Plot What Plot/Porn Without Plot, Sorry Not Sorry, Sticky Sexual Interfacing, Voyeurism, one shot that could possibly grow longer
Language: English
Status: Completed
Published: 2016-01-10
Updated: 2016-01-10
Packaged: 2018-05-12 23:14:47
Rating: Explicit
Warnings: Creator Chose Not To Use Archive Warnings
Chapters: 1
Words: 3,147
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/5684908
Author URL: https://archiveofourown.org/users/Dragonmaster/pseuds/Dragonmaster
Summary: <blockquote class="userstuff">
              <p>What happens when a Decepticon falls into the Underground?  Or, more importantly, what happens when Mettaton and Knock Out discover each other?  Spoilers -- hot fragging ensues.  NOT SORRY.</p>
            </blockquote>





	Beautiful Together

**Author's Note:**

  * For [BasslineRaver](https://archiveofourown.org/users/BasslineRaver/gifts).



> Just a stupid one-shot featuring an old flame (Knock Out) and a newly discovered hottie (Mettaton). Might continue this if I get inspired (or someone out there has an idea to expand on this crack AU).
> 
> No game spoilers except perhaps Mettaton's EX form. 
> 
> I HAVE NO REGRETS.

Asgore had given the monsters of the Underworld strict orders on what to do if they encountered a human – kill it, and bring its SOUL to him. And most monsters were only too happy to oblige. Humans were proven to be dangerous, a threat to even the strongest monsters. And if bringing their SOULs to the King could eventually break the barrier and help them win their freedom… then so be it. In the minds of monster-kind at large (though there were certainly detractors from that mindset), the only good human was a dead human.

There was absolutely no protocol for what to do when a robot fell beneath the mountain, however… and the arrival of a Decepticon in the Underworld had the entire kingdom in a frenzy.

Knock Out had zero inkling of how much of a fuss his sudden arrival had just caused. All he knew was that his finish was scuffed and he was absolutely filthy, what with the dirt jammed in his hubcaps and undercarriage and the gaudy organic plant life caught in his joints. When he finally caught up with those wretched human brats who had lured him on such a merry chase up the mountain, he was going to crush them flat.

The crimson mech plucked a last shred of leaf out of his elbow joint as he made his way out of the ruins. Curse it all! He’d almost had that annoying little girl in his claws, too. Then the rocky ground had given way under his feet, and he’d awakened to find himself lying in a patch of flowers… and had it been his imagination, or had one of them been talking to him?

 _Oh, stop it,_ he told himself with a shake of his head. _Flowers don’t talk. And you’re no simpleton Vehicon to be having such flights of fancy._ Still, he couldn’t shake the feeling that something strange had happened… that the flower had tried to actually initiate a conversation, only going quiet when it had realized he wasn’t human, wasn’t even organic…

 _That’s quite enough._ He shook his head again and pressed forward, snow crunching under his pedes. Bad enough to fall under a mountain and find yourself in a set of ruins that were suspiciously (if conveniently) Cybertronian-sized – either that or he’d somehow shrunk upon entering whatever-this-place-was, which wasn’t outside the realm of possibility. No, now he had to be fantasizing about holding conversations with the local flora. Primus, he needed a CPU scan when he found his way out of here.

***

Even though few monsters still inhabited the Ruins, word spread fast from the abandoned city to Snowdin, and from there to Waterfall, Hotland, and finally to New Home itself. And as monster-kind evacuated their cities and the Royal Guard armed itself against the inevitable, one not-so-brave scientist decided that the only way to defeat a mechanical intruder was to send one of its own kind after it – fighting fire with fire, as it were. She only hoped her creation could hold its own against another mech.

To say the encounter between Knock Out and his erstwhile assassin didn’t go as planned was an understatement of epic proportions.

Mettaton remained still, huddled in the grass in his boxy computer form, watching the intruding mech as he bathed himself in one of the many cascades that gave Waterfall its name. He knew full well he was playing voyeur right now – ironic, wasn’t it, that for once he’d be the viewer and not the entertainer – but frankly, he didn’t care. For once he had a show before him that was just as entertaining – and attractive – as he was, and he’d be a fool to pass it up.

The crimson mech leaned his head back, shutters sliding down to obscure his exotic scarlet optics, letting the water run in rivulets down his crested helm and silver-white face plates. Had he been in his humanoid form, Mettaton would have bitten his lip at the sight of that ivory neck displayed for his viewing pleasure, marked with striking black stripes and just begging to be touched. The mech turned his head from one side to the other, relishing the feeling of the cleansing liquid… and inadvertently displaying his sculpted crests and magnificent neck to full effect.

The rest of his chassis was no burden to look at either, especially now as the filth of Snowdin and the garbage dumps sloughed away. His armor gleamed a handsome crimson in the crystalline light of the Underground, all sleek curves and tapered points. His clawed hands, elegant but by no means delicate, moved with expert grace over his plating to rub away the grime of his journey. Here and there plates of silver or gray accented his ruby armor, highlighting the beauty rather than detracting from it. His entire frame and manner bespoke a being with grace as well as power, a being who prized style as well as strength. 

That, and it was quite obvious that this mech took excellent care of himself. There was hardly a scratch or a dent to be seen in his plating, and his joints moved smooth as oil, without so much as a squeak or a groan. And he was a thorough bather, hands delving into his shoulder pauldrons and behind his knee guards to scrub out hidden dirt, tapered fingers inspecting every nook and cranny…

The scarlet optics opened slightly, giving the mech the half-lidded look of a predator who had just sighted prey he knew couldn’t escape his claws. A smug smile played on his lip plates.

“Enjoying the show?” That voice was rich and deep, and came obscenely close to making Mettaton melt into a pile of slagged circuitry right there. “I know you’re there.”

Mettaton remained silent. How…

The mech gave a liquid chuckle. “I haven’t survived this long as a warrior without learning to tell when someone’s got an optic on me. Though it’s quite nice to have an appreciative audience.” He tilted his helm back to rinse away the last of the grime. “You haven’t ambushed me, so I can only assume you’re harmless. So come on out now. Don’t be shy.”

Mettaton replied with an electronic snort. “I’m the Underground’s premiere showman. I’m not shy in the least.”

“Then come out where I can see you,” the mech replied, stepping out from under the waterfall. “I don’t bite. Hard.”

There was really no point in hiding any longer. Mettaton wheeled his way out of the tall grass, standing before the intruder. He only came up to the mech’s chest, though that didn’t concern him overly. The news that this robot confirmed himself as a warrior was a bit more worrisome, and he wondered briefly how many XP he had racked up…

“Hmmm… not what I expected.” The mech frowned as he looked the monster-built robot up and down. “Pity. You had such a nice voice, I expected a stunning chassis to accompany it.”

Mettaton drew himself up straighter, indignant for a moment, before remembering he was still in his computer form. He allowed himself a smug laugh of his own and gestured to his chassis with one hand.

“Oh, if it’s _stunning_ you want, then _stunning_ you shall have!” he declared. He reached behind his back with one hand, feeling for the switch he knew Alphys had installed there. This was much easier when someone else helped him, but he wasn’t about to turn his back on this mech yet, let alone ask him to mess with his controls…

 _There._ A nudge of the switch, the still-unfamiliar-but-not-unpleasant sensation of unfolding and reconfiguring…

The mech’s jaw dropped, and his shutters widened to expose optics that blazed with shock… and, Mettaton was pleased to note, undisguised awe and admiration. He let a smirk play out on his lips and struck a pose that he knew would show off his humanoid chassis to its greatest effect. Jet-black and magenta, shining with new paint and a fresh polish, he gleamed with a luster that topped even the scarlet mech’s chassis, and the sleek curves of his own chassis were fair competition to his newfound rival’s. He tossed back his black synthetic hair and offered a genuine smile.

“Oh. My.” There was no disguising the approving tone in the mech’s voice. “Magnificent.”

“Did you expect anything less?” he chuckled. “But how rude of me, I haven’t introduced myself. Mettaton, entertainer robot and anti-human defense for the Underground, at your service.”

The mech’s optics flashed with hatred at the mention of _human_ , and Mettaton wondered if Alphys’ theory that humans had built and sent this robot to wipe them out was wrong after all. But a smile soon replaced his scowl. 

“You’re quite the show-mech,” he noted. “Well then, greetings to you, Mettaton! Knock Out, Decepticon medic and warrior, at my own service and no one else’s at the moment.” He offered a regal bow. “Delighted to make your acquaintance.”

“Oh, the pleasure is mine,” Mettaton purred. “You are a magnificent specimen yourself, you know. It’s a pity I’ve been sent to deactivate you.”

“Oh?” Knock Out arched an optic ridge. “Such a shame… though if the powers that reign here must send an assassin, at least they had the good grace to send such a handsome one.” Out of nowhere he drew a pronged staff, spinning it in one hand, the forked end blazing with electricity until it formed a spinning circle of blue lightning. “It would be a shame to destroy such a perfect specimen of mech-kind, but if you leave me no choice…”

Mettaton raised his hands, readying an attack of his own. “Oh, there is always a choice, Knock Out – such a fitting name, by the way. Down here, you can fight your way through… or there is always the option of mercy.”

Knock Out crouched in a battle stance. “Mercy? Decepticons aren’t given to showing mercy to our foes.”

“Perhaps… but I’m not precisely a foe, am I?” The two mechs circled one another, panther-like, each waiting for the other to strike first. “I have no desire to fight you, and clearly you have no desire to fight me. I’m no foe of yours – I’d be perfectly willing to let you pass on by, provided you left me unharmed. But I need a little incentive.”

Knock Out’s stance relaxed just a touch, and he cocked his head to one side. “Incentive? A bribe, then? Why, whatever would your superiors say?”

Mettaton grinned. “Not precisely a bribe. Just incentive to let you go on unmolested. Or, perhaps, to put it another way… molested in the most glorious of ways.”

His optics flared again, but just as much in delight as in shock. “Why… I believe I like you, Mettaton!” The electric glow flickered out, and he crouched to lower his staff to the floor. “Mmm, I like you very much indeed.”

Mettaton’s smile threatened to crack his face plates. This encounter had just gotten interesting in the best of ways. 

***

Part of Knock Out’s CPU wondered if, at some point, Mettaton’s creator had happened upon the remains of a Cybertronian somewhere in the Underground, and been inspired by that to build him. It would certainly explain how his interfacing equipment so closely resembled that of his own kind, and was so delightfully compatible with his own. Perhaps he would have to track down this Alphys creature and thank her personally once they were done here… though that wouldn’t be for a good long while yet.

The rest of his CPU was ablaze with pleasure… no, pleasure was too tame a word for it. Ecstasy fit the bill far better. Who would have thought an organic-built mechanism could be so good at this?

Mettaton arched back, head resting on Knock Out’s shoulder, mouth open as he cycled air heavily through his fans. The black-and-pink mech rode his spike, back pressed against Knock Out’s chest, one of the medic’s clawed hands holding him around the waist while the other reached up to tangle in the silky mass of his synthetic hair. Strange… he’d always regarded hair as tacky and disgusting, but Mettaton wore it well, adding to his appeal.

“O-oh yessssss,” Mettaton purred, reaching up to grab the back of Knock Out’s helm. “Gods… I must thank Alphys for this upgrade…”

“Mmmm… pass MY thanks along,” he murmured, letting himself be pulled down for a blistering kiss. He rocked up into Mettaton’s valve, relishing the slick heat around his spike, the smoothness of plating pressed against his own. How long it had been since he’d had the chance to ‘face another as attractive as himself, one who took as good of care of his chassis as he did. Breakdown was always loving and attentive, and an absolute wonder in the berth, but THIS… 

“Up,” he groaned, clamping his partner’s waist in both hands and raising him up off his spike. “I want… to finish… face to face.”

“Happy to oblidge, my dear,” Mettaton replied, and he turned around and settled himself back in the scarlet mech’s lap. His own spike stood proudly between them, silver with luminescent pink highlighting, and Knock Out had the insane urge to kneel before Mettaton and pleasure it with his own lips and glossa. He restrained himself with difficulty – time enough to indulge in that, time enough…

Mettaton bucked and arched as Knock Out pulled him down onto his spike again. “Aaaah!”

“Mmhhhh… did… did I hurt you?” He prided himself in his skill as a lover, and knowing he might have damaged a partner, however accidentally, did not sit well with him.

Mettaton shook his head, his shining hair disarrayed in a manner that seemed far more erotic than messy. “Don’t you dare stop now.”

“Oh, I have no intention of stopping.” He lowered the black-and-pink mech to the floor of the cavern, on a matt of moss that seemed to have grown there just for the purpose of trysts like this. Had Mettaton taken other partners here, he wondered… 

Then all coherent thought fled as he thrust into the mech with earnest, gently at first, then harder and deeper once he was sure Mettaton could take it. Mettaton wasn’t a passive lover – he pulled Knock Out down to fasten his lips on his neck, sucking and biting, those wonderful legs of his wrapped around his waist. Emboldened by that passionate reaction, Knock Out let his own hands explore as he fragged his partner, caressing his thighs, his sides, his glorious aft… running his claws through his hair…

His entire chassis convulsed as the charge building in his core finally hit its peak, sending a wave of sensation flooding his neural net. He bucked and screamed with the force of his overload, his hips continuing to piston into the mech beneath him. As if in answer to his climax, Mettaton let out a wild cry and arched against him, arms and legs wrapping tightly around him, his own chassis shaking with overload or something very close to it. Together they moved, perfectly in synch, trying to prolong the ecstasy as long as they could.

Finally Mettaton collapsed backward, panting, arms splayed out over his head. He gazed up at the Decepticon medic with a look of lazy, sated content. Knock Out, for his part, had to brace his hands in the moss to keep from collapsing on top of him with all his weight. He felt drained, exhausted, but utterly exhilarated as well, in the way only a good interface session could make him.

“Oh yes,” Mettaton groaned, shuttering his optics. “Truly magnificent.”

“Mmmmm… you’re quite good yourself.” Knock Out leaned down and let his lip plates brush against his partner’s. “Perhaps I should see about taking you home with me. You could use an appreciative audience.” That was no jest or flattery – Mettaton was wasted down here with only organic creatures for company. On the Nemesis, however, Knock Out could give him the attention he deserved. 

Mettaton cracked an optic shutter open, smirking up at him. “Oh? Who said you were leaving?”

Knock Out frowned, anger briefly slashing through his pleasure. If this was a trap…

“I think, before I can let you pass, I need a bit more… incentive.” Mettaton chuckled. “Unless you haven’t the stamina to try again?”

His anger faded, arousal flooding his chassis again as if he hadn’t just fragged the daylights out of the only mech on this planet as beautiful as he was. “Oh, I’ll show you stamina.” He rolled over, dragging Mettaton with him, so he lay on his back with the black-and-pink mech straddling his hips. “Though be careful. You might not be able to walk out of here by the time I’m done with you.”

“Oh, you promise?” crooned Mettaton, one hand reaching behind him to find Knock Out’s valve. He jerked and bit his lip at the touch, spreading his legs to give him better access. Primus below, this was worth falling under a mountain for.

***

Just outside the cavern, three monsters huddled behind a boulder, not daring to peer inside. The clang of metal on metal and the ringing cries echoing from within were enough to deter them.

“Cripes, how long does it take to rip a robot limb from limb?” Undyne growled. “I’d’ve had the loser beaten to scrap metal by now.”

“It sounds like that robot’s putting up a good fight,” Papyrus pointed out. “PLEASE can’t I go in there and help Mettaton out, Undyne? I can prove myself in there!”

“What, by throwing word searches and bones at him?” Undyne snapped. “Leave this to someone who knows what he’s doing.”

“Come on, how am I ever going to join the Royal Guard if I never get a chance to prove myself?” Papyrus whined.

Sans cocked his head to one side as Mettaton’s shout rang from the cavern, his eternal smile seeming to take on a knowing tilt. “Sounds like ol’ Metta is getting pounded really good in there.”

“I knew it!” Papyrus crowed, Sans’ pun flying right over his skull. “Let’s go in there and give him a hand!”

“I’ll give you a hand if you don’t shut up,” Undyne griped. “Right in the kisser! Hey Mettaton, hurry it up in there, will you? We’ll help you bury the body when you’re done!”

Sans sighed and pulled a quantum-physics-joke book out of his coat to pass the time. He knew full well what was going on in that cavern, and he had no desire to see it himself – if he wanted to see robots screwing around (hey, that wasn’t a half-bad pun) he could go to the Internet. For now he’d just wait until either Mettaton or his newfound buddy got bored, and then see where the timeline took them from there. However it went, it was bound to get interesting…


End file.
